It is well known in the art of oil production that only a portion of the oil present in subterranean oil reservoirs can be recovered by direct methods, also called primary recovery techniques. A portion of the oil which is not recoverable by primary recovery techniques, can be recovered by secondary recovery techniques. The latter techniques include the use of a displacement fluid e.g. water, which is injected into the oil reservoir or into the vicinity of the reservoir.
If after application of the secondary recovery techniques an appreciable amount of oil is still retained in the reservoir, it is still possible to recover a part of that oil from that reservoir.
This oil, in the form of immobile, capillary-trapped droplets, can be mobilized by injection of suitable surfactant solutions; these interact with the oil to form a micro-emulsion that reduces the capillary trapping forces to a very low level. The process is called surfactant flooding.
Once mobilized, the oil forms a growing bank that leaves almost no oil behind in the flooded part of the reservoir. Since the oil bank precedes the surfactant bank it is not necessary to inject surfactant continuously throughout the flood. So when a certain volume of surfactant solution has been injected, it may be followed by a cheaper fluid, such as viscous water; and later water alone. The injection of the surfactant, viscous water and water involves the displacement of oil to the production well. The micro-emulsion on its turn is displaced in the direction of the production well by the viscous water. Consequently a number of moving zones and banks is developing in the oil reservoir and as long as the micro-emulsion is intact, oil can be recovered.
Surfactants are soaps or soap-like chemicals. Their molecules consist of a hydrophilic part, attracted to water, and a lipophilic part, attracted to oil. Because of this amphiphilic nature, even at small concentrations, they can greatly reduce the interfacial tension between oil and water and form micro-emulsions.
It is a problem to recover the oil from deeper reservoirs, since at ever increasing depths the temperature becomes higher and the micro-emulsion breaks down in oil and water, whereby the oil is again trapped in the pores. The micro-emulsions formed by oil, surfactant and water are thus not stable enough to withstand the higher temperatures.
It is an object of the invention to find surfactant compositions which are able to form micro-emulsions which are stable at high temperatures as well as at low temperatures and which over a broad temperature range are in equilibrium with their environment and do not deteriorate.